UNIVERSITY OF CINCINNATI Date:___________________ I, _________________________________________________________, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair: _______________________________ _______________________________ _______________________________ _______________________________ _______________________________ The impact of avian predation on the brush-legged wolf spider, Schizocosa ocreata (Hentz), and anti-predator responses to avian cues. A thesis submitted to the Division of Research and Advanced Studies Of the University of Cincinnati In partial fulfillment of the requirements for the degree of MASTER’S OF SCIENCE (M.S.) In the department of Biological Sciences of the McMicken College of Arts and Sciences 2007 By Anne K. Lohrey B.A. Miami University, 2003 Committee: Dr. George W. Uetz, Chair Dr. Kenneth Petren Dr. Ann Rypstra Abstract This research aimed to quantify the potential for avian predation on Schizocosa ocreata wolf spiders in the field and its impact on spider behavior. In a field study, enclosures that excluded birds had a higher proportion of spiders remaining at the end of the experiment than enclosures that allowed birds access. Additionally, observational data confirmed that some bird species seen active at the study site eat spiders presented in feeding trials. These data suggest that bird predation impacts survival of S. ocreata in the field. In the laboratory, I tested spiders‘ recognition of and behavioral responses to sensory cues indicating the presence of avian predators. Courting male S. ocreata responded to avian acoustic stimuli (bird calls) with anti- predator behavior, which supports the hypothesis that bird predation limits survival of S. ocreata and may be an important selective factor on the evolution of behavior in this species of wolf spider. i ii Acknowledgements I would like to first thank my advisor, Dr. George W. Uetz, for his guidance throughout this project. He has challenged me to become a better writer, presenter and thinker through this experience. My committee members, Dr. Ken Petren and Dr. Ann Rypstra, were instrumental in my success as well. They were always available and helped guide me through this process with great ideas to improve my project and make it more cohesive. Thank you to my fellow graduate students, E. Galbraith, J. Gibson, S. Gordon, J. Johns, B. Moskalik and J. Rutledge for being such great sounding boards and providing much needed comic relief. I’d also like to thank the many undergraduate students who helped make this research possible, especially J. Allen, I. Huang, M. Skelton, J. Smith and A. Stein. My family also deserves much gratitude for supporting me through this endeavor. They always offered a listening ear for the difficult times and helped me celebrate my successes. My parents instilled in me, by example, a sense of integrity, a strong work ethic and impressed on me the value of wisdom as well as knowledge. Finally, I would like to thank my best friend and partner in life, Jeremy Gibson, who has been unwavering in his support and encouragement. iii Table of Contents Abstract i Acknowledgements iii List of Tables and Figures 2 General Introduction 4 Study Organism 4 Objectives and Hypotheses 6 Chapter Page I. Potential for avian predation on Schizocosa ocreata (Hentz) wolf spiders (Araneae: Lycosidae) 12 Abstract 13 Introduction 13 Methods 16 Results 18 Discussion 20 References 23 II. Recognition of and response to avian predator cues in Schizocosa ocreata (Hentz) wolf spiders (Araneae: Lycosidae) 31 Abstract 32 Introduction 32 Methods 35 Results 38 Discussion 39 References 43 Conclusions and Future Directions 52 References 55 Appendix: Distance over which S. ocreata can be detected visually by birds in the field 56 1 List of Tables and Figures Tables Table 1.1. Ground foraging bird species observed at the field site during both anecdotal observations and timed focal observations (9 days; 15 min./day). Table 1.2. Mean proportion of spiders remaining at the end of the experiment for each treatment. Table 1.3. Two-way ANOVA of proportion of spiders remaining at the final census point. Table 1.4. Two-way repeated measures ANOVA of the proportion of spiders remaining at each census point (duration). Figures Figure 1.1. Overlap in duration of S. ocreata breeding season, breeding season of Ohio birds and the enclosure study. Figure 1.2. Mean proportion of spiders remaining at the end of the experiment for each treatment. N = 19 (Uncovered), N=17 (Covered). Figure 1.3. Mean proportion of spiders remaining at each census point (duration). N=10 each. Figure 2.1. Percentage of each behavior (locomotion, tap, stop, groom) following the stimulus (a. acoustic, b. visual, c. seismic, d. control) for each type of cue (N=10 per treatment group). Figure 2.2. Average latency to return to courtship after presentation of each type of stimulus N=10 (Seismic), N=9 (Acoustic), N=9 (Visual), N=9 (Control). Figure 2.3. Preliminary results from 2006. Percentage of spiders given each stimulus that responded with anti-predator behavior (N=12 per treatment group). Figure 2.4. Percentage of spiders given each stimulus that responded with anti-predator behavior. N=21(Carolina Wren, Northern Cardinal, House Finch), N=16(Katydid), N=19(White Noise). Figure 2.5. Percentage of spiders given each stimulus type (Bird Call, Non-threatening Sound) that responded with anti-predator behavior. N=63 (Bird Calls), N=35 (Non-threatening Sounds). Figure 2.6. Average latency to return to courtship after presentation of stimulus (in seconds). N=19 (Carolina Wren), N=21 (Northern Cardinal), N=18 (House Finch), N=16 (Katydid), N=20 (White Noise). Figure 2.7. Average latency to return to courtship after presentation of stimulus (in seconds). N=58 (Bird Calls), N=36 (Non-threatening Sounds). 2 Figure 2.8. Spectrum analysis for Katydid call compared to bird calls (House Finch and Northern Cardinal). Note that the frequency scale varies between the Katydid call (a) and the bird calls (b and c). Figure 3.1. Two-dimensional representation of the average distance to the first visual obstruction from a point on the leaf litter, measured in two sites: Cincinnati Nature Center (a) and New Richmond, Ohio (b). Measurements for the New Richmond site (spring, 2007) were taken four times (May 12, 15, 22 and 20) over the breeding season. 3 General Introduction Elaborate male courtship signals and displays have often been explained by sexual selection, since such traits may increase conspicuousness of males to females and/or serve as indicators of male quality, thereby increasing mating success (Andersson, 1994; Johnstone, 1995). However, such signals may incur predation costs, as elaborate male traits make the trait- bearer more conspicuous to predators as well as to females (Zuk & Kolluru, 1998; Haynes & Yeargan, 1999; Kotiaho, 2001). The respective benefits and costs of sexual and natural selection can result in a trade-off between them (Tuttle & Ryan, 1981; Endler, 1992; Andersson, 1994; Zuk & Kolluru, 1998; Basolo & Wagner, 2004; Husak et al, 2006). For example, male Calopteryx damselflies with increased melanization of wing patches, a sexually selected trait, suffer higher mortality due to predation (Svensson & Friberg, 2007). Likewise, bright body coloration in male guppies (Poecilia reticulata) increases male mating success, but brighter males incur greater direct fitness costs due to predation (Godin & McDonough, 2003). Female Hygrolycosa rubrofasciata wolf spiders prefer males with higher courtship drumming rates; however, such males are subject to increased risk of predation based on drumming rate (Kotiaho et al, 1998, Lindstrom et al, 2006). Study organism Schizocosa ocreata (Hentz), a wolf spider commonly found in the leaf litter of eastern deciduous forests (Cady, 1984), has a spring breeding season. The mating system of this species has been described as scramble competition polygyny and females are unlikely to mate with more than one male (Norton & Uetz, 2005). Males mature in late March or early April (about two weeks prior to females) and become increasingly active on the surface of the leaf litter until 4 the end of the breeding season. In mid-summer, eggsacs hatch and spiderlings overwinter to mature the next spring, completing the life cycle. During courtship, males engage in a complex multi-modal display consisting of both seismic and visual components upon detection of female dragline silk and associated chemical cues (Stratton & Uetz, 1981, 1983, 1986; Scheffer et al., 1996; Uetz, 2000; Uetz & Roberts, 2002). Seismic communication consists of both substrate-borne stridulation produced in the tibio-tarsal joint and percussion of the abdomen and chelicerae against the substrate. Some attributes of the seismic signal have been found to influence female receptivity (Gibson & Uetz, in press). In addition, visual communication consists of active leg-waving displays of the first pair of legs. Associated tufts of bristles on the tibiae of legs I may serve as a decoration, increasing male mating success (Scheffer et al, 1996; Hebets & Uetz, 2000; Uetz, 2000; Uetz & Roberts, 2002; Delaney et al, in press) Leg tufts, while serving to make male courtship displays more conspicuous and/or attractive to females, also increase detection by visually-hunting predators such as other spider species, anurans and/or avian predators with which S. ocreata occur (Wise & Chen, 1999a,b; Pruden & Uetz, 2004; Roberts et al, 2006). Digitally manipulated S. ocreata video images have been used to demonstrate that larger leg tufts elicit a faster predatory response by a jumping spider (Phiddipus clarus), another wolf spider (Hogna helluo) and a vertebrate predator, the American toad (Bufo americanus) (Pruden & Uetz, 2004; Roberts et al, 2006; Roberts & Uetz, unpubl.). Reflectance measurements have shown that S. ocreata leg tufts as well as lateral parts of the prosoma and opisthosoma contrast highly with the spectral qualities of the leaf litter background (Clark, Roberts and Uetz, unpubl.).